In the operation of heat exchangers, such as furnaces or boilers, various gases are produced such as nitrogen oxides (NO.sub.x). Depending on the type of fuel being burned, two types of nitrogen oxides can be formed. Fuel bound NO.sub.x is formed as a result of nitrogen being present in the fuel itself, i.e., in fuel oils. During combustion, the nitrogen is released and quickly reacts with the oxygen in the combustion air to form NO.sub.x. The reactions to the fuel bound NO.sub.x are not particularly temperature-dependent. Thermal NO.sub.x is formed, on the other hand, when high combustion temperatures break down the nitrogen gas in the combustion supporting air to atomic nitrogen. When this occurs, the atomic nitrogen will very quickly react with oxygen to form thermal NO.sub.x.
If natural gas is employed as the furnace or boiler fuel, only thermal NO.sub.x is formed, because clean natural gas does not contain any nitrogen containing compounds. On the other hand, both thermal and fuel bound NO.sub.x are formed when burning fuel oils.
The production of NO.sub.x by the burning of fuels in the operation of boilers and furnaces is potentially damaging to the environment. Accordingly, various environmental emissions standards are being imposed by various governmental authorities and agencies to regulate and to suppress the formation of nitrogen oxides during operation of boilers and furnaces. Various techniques have been utilized in the design and operation of boilers and furnaces to meet those regulations.
For example, it is known that burning a hydrocarbon fuel in less than a stoichiometric concentration of oxygen will produce a reduced amount of CO and H.sub.2. This concept is utilized in a staged airtype low NO.sub.x burner where the fuel is first burned in a deficiency of air in one zone to produce an environment that suppresses NO.sub.x formation, and then the remaining portion of the air is added in a subsequent zone.
Staged fuel also has been suggested for suppressing the NO.sub.x formation. In staged fuel, the air and some of the fuel is burned in the first zone and then the remaining fuel is added in the second zone. The presence of an overabundance of air in the first reaction zone acts as a dilutent, thus lowering the temperature and suppressing NO.sub.x formation. It is also known to recirculate flue gas to lower flame temperature and reduce NO.sub.x formation.
However, each of these prior art processes has certain inherent deficiencies and associated problems which have lead to limited commercial acceptance. For example, when burning fuel in a sub-stoichiometric oxygen environment the tendency for soot formation is increased. The presence of even a small amount of soot will alter the heat transfer properties of the heat exchanger surfaces downstream from the burner. Also, flame stability can be a critical factor when operating a burner at significantly sub-stoichiometric conditions. Moreover, many of the prior processes and systems have been complicated and expensive to build, install, use and maintain and require extensive modifications of standard furnaces, boiler and fuel burners.
Examples of prior heat exchangers and burners are disclosed in the following U.S. Pat. Nos.: 2,532,214 to Willenborg; 3,146,821 to Wuetig; 3,369,587 to Taubmann; 3,797,989 to Gordon; 3,827,851 to Walker; 3,859,935 to Walker; 4,245,980 to Reed et al.; 4,347,052 to Reed et al.; 4,380,429 to LaHaye et al.; 4,445,843 to Nutcher; 4,483,832 to Schirmer; 4,505,666 to Martin et al.; 4,575,332 to Oppenberg et al.; 4,618,323 to Mansour; and 4,629,413 to Michelson et al, the disclosures of which are hereby incorporated herein by reference.
This invention addresses these problems in the art, along with other needs and problems which will become apparent to those skilled in the art once given this disclosure.